Tumor oxygenation plays a critical important role in tumor growth, invasion, and therapy. Tumor tissues usually produce endothelial growth factors (such as VEGF) that stimulate new growth of blood vessels to meet the high nutrient demands of the fast growing tumor cells. Tumor cells have different characteristics as a function of their distance from the blood vessels and can be typified by their degree of oxygenation: the most viable cells are well perfused and oxygenated, the hypoxic cells are poorly oxygenated, and the necrotic center contain dead cells as a result of oxygen deprivation. These tumor cells have different genetic and molecular characteristics and possibly different response to various therapeutic interventions including radiation and chemotherapy. Currently, invasive microelectrode approach is used as a gold standard measurement of tumor oxygenation. Crucial is the selection of the sampling locations with this method since artifacts may be generated when multiple electrode measurements are required around the same tissue location. We propose a novel molecular imaging approach to map tumor oxygenation in vivo based on an attenuated strain of Salmonella typhimurium that selectively accumulates and proliferate in tumor tissues. Myoglobin will be genetically engineered into the Salmonella bacteria as an oxygen sensor. Since Salmonella can penetrate into the hypoxic areas in the tumor, the method will give a full mapping of tumor oxygenation. In this R21 proposal, we will develop a novel molecular imaging methods to map tumor oxygen tension in murine tumor models using the attenuated Salmonella typhimurium strain that over-expresses the oxygen sensing protein myoglobin. Deoxymyoglobin can be measured directly in vivo using a signature peak at -80 ppm in the proton spectroscopy for oxygen tension calculations, or indirectly in high-resolution tumor tissue water imaging whereas deoxymyoglobin functions as a relaxation contrast reagent. This novel measurement of tumor oxygen tension will be applied to observe the primary and metastasized tumors. The methods will have numerous applications in preclinical development of anticancer drugs considering physiological parameters in a particular tumor microenvironment. Since Salmonella typhimurium is currently in phase I clinical trials, the project may have a major impact in the future diagnosis of both primary tumors and distant metastasis, as well as therapeutic planning and monitoring of the cancer clinical outcomes. [unreadable] [unreadable]